In nitride UV LED structures, high efficiencies, herein defined as achieving a high light output per unit of energy input is important. One method of improving LED efficiency is by improving the extraction efficiency of light from the LED structure.
In forming a UV LED structure over a typical sapphire substrate, a thin template layer is formed between the substrate and the n-type AlGaN layer of the LED structure. This thin template layer helps match the substrate lattice structure to the n-type AlGaN layer lattice. In some cases the template layer may include a number of dislocations that do not propagate into the n-type AlGaN layer. These dislocations facilitate the relief of strain resulting from the mismatch between the lattices. However, the template layer also has an unintended and undesirable side effect. In particular, the smooth interface between the substrate and the template layer and a parallel second smooth surface between the n-type AlGaN layer and the template layer work together to form a waveguide. Thus the template layer can act as a waveguide that channels light away from the direction of the intended LED output. Channeling light away from the intended LED direction results in lost photons, and a corresponding decrease in LED efficiency.
Thus a structure and/or method is needed to reduce this channeling effect and thereby improve LED light output.